Sleep must be very important to all animals because it justifies shutting down consciousness, closing off from the external world, reducing input from sensory systems, and risking death from predators. Why would any sensible creature do this? Yet across millions of years of evolution, the mechanisms of sleep were conserved among vertebrates, arthropods, and nematodes. A recent study investigated how far back in time the first sleep occurred.
The scientists studied an animal from the phylum Cnidaria that includes hydra, jellyfish, sea anemones, and corals. These animals lack a centralized nervous system; they function mostly as a thick sheet of cells embedded with a network of nerves that innervates most parts of their body. Earlier research had demonstrated that animals of the cnidarian species undergo daily periodic quiescence that displays the 24-hour activity-rest rhythms that we would expect to observe in any organism that evolved on a planet spinning every 24 hours in the presence of a sun. Surprisingly, hydra sleep is not tied to an internal clock. Apparently, internal clocks evolved later.
The current study used Hydra Vulgaris as the cnidarian model of sleep. Hydra consists of only two cell layers shaped like a hollow tube. The adults attach themselves to a rock surface with their open end facing outward. Hydra are carnivorous and eat small metazoans and even insects. They capture their food by paralyzing and killing by means of nematocysts which discharge toxins, such as phospholipase A2 (similar to that used by snakes) into their prey. Phospholipase A2 essentially dissolves the cell walls of the prey leaving the juicy center.
The recent study discovered numerous similarities between the neurochemistry of hydra and human sleep mechanisms. For example, adding melatonin to the fluid surrounding the hydra dose-dependently increased their daily sleep amount and the number of sleep bouts. Melatonin is a hormone that our brain produces in response to darkness that helps with the timing of our circadian rhythms, such as sleep onset. This is why melatonin is most helpful in combating the effects of jet-lag. Apparently, the response to melatonin preceded the evolution of internal clocks.
One of the most interesting discoveries was that the hydra responded to the neurotransmitter GABA in the exact way that the human brain responds. Stimulating GABA receptors promoted sleep. GABA is one of the two most common neurotransmitters in the human brain. Apparently, its importance as a neural inhibitor has been conserved across five hundred million years of evolution. The GABA receptor is also the target of other drugs that humans use to help them sleep, including alcohol, barbiturates, and benzodiazepines. This raises another important question: Can hydra get drunk?
Exposure of the hydra to the precursor of dopamine production also induced sleep. The authors found this effect surprising because dopamine normally causes humans to become more alert and suppresses sleep via its ability to inhibit norepinephrine. This finding confirms that these ancient creatures use enzymes in their neurons that are similar to those found in the human brain. The authors speculated that the dopaminergic sleep-regulatory pathway may have switched from sleep-promoting to wake-promoting during the evolutionary development of the brain. The study also discovered some genes in the hydra that are the ancestral genes that regulate sleep in all animals.
Future studies using animals with even simpler nervous systems, such as the roundworm (with 302 neurons) or Tardigrade (with about 200 neurons), may ultimately elucidate the origin of sleep. We may one day discover that hydra, who can brag of having about 6000 neurons, dream.
© Gary L. Wenk, Ph.D., author of The Brain: What Everyone Needs to Know, (Oxford University Press).
Kanaya, HJ et al (2020) A sleep-like state in Hydra unravels conserved sleep mechanisms during the evolutionary development of the central nervous system. Sci Adv. 2020 Oct; 6(41): eabb9415 PMCID: PMC7541080